Gas flow control elements such as valves and dampers are utilized in a variety of applications including the control of gas flow either into or out of a selected enclosure or other area. Examples of applications where such flow control elements are utilized include fume hood controllers and controllers for maintaining a selected positive or negative pressure in a sealed environment for clean room or containment purposes, respectively.
While in many such applications, system tolerances are such that precise flow control is not required, there are applications, for example high level containment sealed rooms, where precision control of flow is required in order to precisely maintain a desired pressure level within the room. Valves or other flow control elements which, either because of small size or of operating point, are being used to provide low flow rates are generally resolvable within a few cubic feet per minute (cfm), thus providing the desired precision control. However, since the resolvability of such devices frequently varies as a function of flow, precision control to a few cfm is generally not possible in existing systems where larger elements operating at higher cfm are being utilized. A need, therefore, exists for an improved flow control device which permits precision flow control to be achieved over the entire operating flow range of the system, including at relatively high flow rates.
In particular, in a clean room or containment situation where a selected offset in flow between supply and exhaust air flow is maintained to achieve and maintain a selected relatively precise positive or negative pressure level in the room, fine adjustments in flow through the slave or tracking flow controller may be required in order to maintain the desired room pressure. This problem is discussed in greater detail in the parent application. However, existing systems which may be required to move relatively large air volumes for a large sealed room to maintain, for example, a minimum of 10 changes in air per hour, have difficulty in making the small adjustments in exhaust fluid flow necessary to precisely maintain room pressure. A need, therefore, exists for an improved gas flow control device which is capable of providing precise gas flow control regardless of gas flow rate through the controller, and in particular for such a device which can be used in a sealed room/enclosure to permit precision pressure control at high gas flow volumes.